Welteam It isn't in GOS 13 and wasn't in GOS 12 because it never was part of AOSP, simple as that.
Not only is it not in AOSP as Welteam mentioned above, it is not part of Googles Pixel software either.
GrapheneOS is a fully production ready OS and is not and does not work like a hobbyist 'custom ROM' cherry picking features.
Just because something is available in another place regardless if it is Open Source or not there could be licensing, authorship, maintenance issues etc
If you wish to see it and can maintain it, making a PR might be an option.
Building GOS yourself and adding in the patches you wish to use is another.
I agree there are always nice to have features but the core dev team don't have a lot of time to devote to such things outside the core project.
We would love to be more responsive to these things but unfortunately even if listed on the Issue Tracker it will be tagged as a low priority enhancement. To change this we would need the dev team to grow and provide the project the funds for it to do so.
Any and all donations to achieve this would be graciously received and appreciated and if anyone feels they can then they can find details here: https://grapheneos.org/donate